In recent years, ultrasound has received a great deal of attention as a new technique for noninvasive assessment of bone, and numerous attempts have been made to use ultrasonic energy for evaluating the condition of bone tissue in vivo, and thus for defining a stage of development of osteoporosis and assessing bone fracture risk.
In particular, Hoop discloses in U.S. Pat. No. 3,847,141 a device to measure bone density as a means for monitoring calcium content of the involved bone. A pair of opposed ultrasonic transducers is applied to opposite sides of a patient's finger, such that recurrent pulses transmitted via one transducer are "focused" on the bone, while the receiving response of the other transducer is similarly "focused" to receive pulses that have been transmitted through the bone. The circuitry in Hoop is arranged such that filtered reception of one pulse triggers the next pulse transmission; the filtering is by way of a bandpass filter, passing components of received signals in the 25 kHz to 125 kHz range only; and the observed frequency of retriggering is believed to be proportional to the calcium content of the bone. Thus, Hoop is not concerned with anything more than what he perceives to be transit time for pulses in the indicated band.
Pratt, Jr. deals with establishing, in vivo, the strength of bone in a live being such as a horse. In U.S. Pat. No. 4,361,154, the inventor solves the problem posed by measuring transit time from "launch" to "reception" of pulses of 0.5 MHZ and 1.0 MHZ through the bone and soft tissue, and from measurement of pulse-echo time, to thereby derive a measurement of transit time through bone alone. A data bank enables the evaluation of the meaning of variations in measurements of the transit time which is deduced to be correlated with propagation velocity through each measured bone. U.S. Pat. No. 4,913,157, also granted to Pratt, Jr., operates on the same general principle of transit time/velocity deduction, using the later preferred frequency of 2.25 MHZ as the base frequency of pulsed "launchings" and a technique of matched filtering/Fourier transform filtering for further analyzing received pulses. The bone-transfer function is purported to be derived from analysis of an average of the received pulses. In his U.S. Pat. No. 4,941,474, the inventor further refines his technique of transit time/velocity deduction, inter alia, by separately determining the ratio of the velocity of his observed "bone signal" to the velocity of his observed "soft-tissue signal" making use of the same technique of filtering set forth in his U.S. Pat. No. 4,913,157.
Palmer et al. disclose in U.S. Pat. No. 4,774,959 a bone measurement system deriving the slope of the relation between ultrasonic frequency and attenuation of a sequence of tone signals. Being in the range of 200 to 600 kHz, the signals are applied to one transducer and received by another transducer. The passage of the signals between the two transducers with and without the intervening presence of a heel bone is compared, with the assumption that the frequency/attenuation relation is a straight line, i.e. of constant slope.
U.S. Pat. No. 4,926,870 granted to Brandenburger discloses another in vivo bone-analysis system which depends upon measuring transit time for an ultrasonic signal along a desired path through a bone. A "canonical" wave form, determined by previous experience to be on the correct path, is used for comparison against received signals for transmission through the patient's bone, while the latter is reoriented until the received signal indicates that the bone is aligned with the desired path. Again, ultrasonic velocity through the patient's bone is assumed to have been determined from measured transit time.
Rossman et al disclose in U.S. Pat. No. 5,054,490 an ultrasound densitometer for measuring physical properties and integrity of a bone, upon determination of transit time, in vivo, through a given bone, in comparison with transit time through a medium of known acoustic properties. Alternatively, the Rossman et al. device compares absolute attenuation of specific frequency components of ultrasound acoustic signals through the bone with the absolute attenuation of the same frequency components through a medium of known acoustic properties. For attenuation measurements, a "broad-band ultrasonic pulse" is recommended and is illustrated as a single spike "which resonates with a broadband ultrasonic emission". The necessary comparisons are performed by a microprocessor, resulting in a slope of attenuation versus frequency in the broadband of interest. The frequencies or frequency ranges are not disclosed. Of note, the ultrasound densitometer disclosed in the Rossman, et al, patent is a two-dimensional array transducer consisting of twelve elements. Each element is activated individually by an ultrasound acoustic signal.
The prior art, exemplified by the above references that have been briefly discussed, proceed on the assumptions that transit time is all-important in assessing acoustic velocity or that only one or a few specific ultrasonic frequencies are significant in the determination of the attenuation versus frequency "slope" of a presumably linear relationship. These approaches have been essentially ad hoc, with no consistent framework within which to analyze data. Despite the fact that a rich variety of information is obtainable from experiments with ultrasound, much of the information has not been used and available, and useful aspects of the data have been ignored.
A step forward in this direction was made by Kaufman et al., who disclosed in U.S. Pat No. 5,259,384 an apparatus and method for quantitatively evaluating bone tissue in vivo. Whereas the prior methods have relied on rather simplistic analyses techniques, the method of Kaufman et al. disclosed in the U.S. Pat. No. 5,259,384 Patent includes iterative subjecting bone to an ultrasonic acoustic excitation signal pulse of finite duration, supplied to one of two transducers on opposite sides of the bone, and involving a signal consisting of plural frequencies in the ultrasonic region to approximately 2 MHZ; the excitation signal is repeated substantially in the range from 1 to 1000 Hz. Signal processing of received signal output of the other transducer is operative (a) to sequentially average the most recently received given number of successive signals to obtain an averaged per-pulse signal and (b) to produce a Fourier transform of the averaged per-pulse signal. In a separate operation not involving the bone, the same transducers respond to the transmission and reception of the same excitation signal via a medium of known acoustic properties and path length to establish a reference signal, and this reference signal is processed to produce a Fourier transform of the reference signal.
The two Fourier transforms in the U.S. Pat. No. 5,259,384 Patent are comparatively evaluated to produce a bone-transfer function, and the bone-transfer function is processed to derive the frequency-dependent specific-attenuation function .mu.(f) and the frequency-dependent group-velocity function v.sub.g (f) associated with the bone-transfer function. Specifically, the frequency-dependent group-velocity function v.sub.g (f) is related to the derivative of the phase of the bone-transfer function, as a function of frequency. Finally, a neural network, configured to generate an estimate of one or more of the desired bone-related quantities, is connected for response to the specific-attenuation function .mu.(f) and to the group-velocity function v.sub.g (f), to thereby generate the indicated estimates of the status of the bone that is being analyzed.
All advantages of the last-mentioned apparatus and method notwithstanding, they do not use statistically optimal techniques and therefore may be subject to substantial inaccuracies. In addition, their implementation with the use of current ultrasound devices is still relatively complex and costly, although simpler than that using X-ray densitometric systems.
In addition to the failure of prior art to use all of the available information from ultrasonic assessment, the prior art has predominantly relied on the use of single element transducers. One notable exception was Rossman, et. al, U.S. Pat. No. 5,054,490 as mentioned supra, although Rossman et. al still use standard ultrasonic transit time and attenuation slope ("BUA") techniques. The use of single element transducers makes it difficult to obtain reproducible ultrasound parameter estimates. As a result, it is more difficult to make accurate and precise estimates of bone density, bone strength, bone architecture, bone quality and fracture risk and to make intrapatient and interpatient comparisons based on such estimates.